Manufacturing-sensitive control of high rotor pole switched reluctance motors

ABSTRACT

A method for controlling switched reluctance machine (SRM) utilizing a SRM control system. The method allows for adaptive pulse positioning over a wide range of speeds and loads. An initial rotor position is provided for the SRM utilizing an initialization mechanism. A pinned point on a phase current waveform is defined during an initial current rise phase of the current waveform. A slope of the current rise is determined as the current waveform reaches the pinned point. The slope is then fed to the commutation module of the SRM control system. An error signal from calculated inductance or current slope is used as an input to a control loop in the SRM control system. The time determining module determines an optimum time signal to fire a next pulse. The optimum time signal is fed to the SRM for turning the plurality of SRM switches to on and off states.

RELATED APPLICATIONS

This application is a 35 U.S.C. 371 national phase application claimingpriority to the International Application PCT/US2018/025609, filed Mar.31, 2018. The disclosure of that application is incorporated herein asif set out in full.

BACKGROUND OF THE DISCLOSURE Technical Field of the Disclosure

The present disclosure relates generally to methods for controllingswitched reluctance machines (SRMs), and more particularly to a methodand system for controlling SRM to enable adaptive pulse positioning overa wide range of speeds and loads.

Description of the Related Art

A switched reluctance machine (SRM) is a simple type of electric motorthat operates by reluctance torque. SRM includes salient rotor andstator poles. There are concentrated windings on the stator but nowindings or permanent magnets on the rotor. These features enable theSRM to achieve very high-speed relative to conventional non-SRM motors.Since there are no windings in the rotor, power is only delivered to thewindings in the stator rather than the rotor, and due to this simplemechanical construction SRMs offer lower maintenance costs relative toconventional electric motors. When current is passed through the statorwindings, torque is generated by the tendency of the rotor poles toalign with the excited stator pole. Continuous torque can be generatedby synchronizing each phase's excitation with the rotor position.Accurate rotor position information is essential for controlling themotor torque.

Several techniques have been proposed for position estimation usinginductance of the active or inactive phase. In most methods, acontrolled signal is utilized and may be applied to the phase winding toestimate inductance and thus determine rotor position without the use ofa position encoder. Certain other methods describe auto-calibration of amotor. One such method describes a sensor-less rotor positionmeasurement system having a digital processor which receives signalsfrom current and flux sensors of the current and flux associated with aphase winding of the machine. The measurement of the current and flux isenabled at a predicted reference rotor position. Current and flux aresampled only once per energization cycle. This method is based onposition estimation methodology, which fails to provide absolute rotorposition information.

Another method describes a circuit for controlling a switched reluctancemotor through indirect sensing of rotor position within the switchedreluctance motor. This method measures time for the current to risebetween two predetermined levels. The measured current rise time can becompared to a desired current rise time to determine whether conductionintervals in the motor phases are in-phase with the position of therotor or are lagging or leading the position of the rotor. However, thismethod utilizes complex algorithms for calculating the current risetime.

Yet another method for controlling a switched reluctance electricmachine includes a switched reluctance electric machine having a sensorgenerating and transmitting a sensor signal indicative of an operatingcharacteristic, a controller operatively coupled to the switchedreluctance motor and the sensor and the controller executing a method.Here, the sensor-less control of SRM is done by injecting a pulse ofvoltage and measuring resultant current in the phase. However, thismethod injects additional voltage pulses for controlling the switchedreluctance electric machine.

There is thus a need for a method for controlling a switched reluctancemachine to achieve adaptive pulse positioning. Such a method wouldreduce manufacturing imperfections and aging effects in the machine.Further, such a method would adjust control parameters for eachindividual machine instead of the entire batch of manufactured machines.Moreover, such a method would provide accurate rotor positioninformation. Such a method would utilize simple algorithms forcalculating the current rise time. Further, such a method would notinject additional voltage pulses for controlling the switched reluctanceelectric machine. These and other objectives are accomplished by thepresent embodiment.

SUMMARY OF THE INVENTION

To minimize the limitations found in the prior art, and to minimizeother limitations that will be apparent upon the reading of thespecification, the preferred embodiment of the present inventionprovides a switched reluctance machine (SRM) control system thatcontrols an SRM and enables adaptive pulse positioning over a wide rangeof speeds and loads. The SRM control system includes an initializationmodule to provide an initial rotor position for the SRM utilizing aninitialization mechanism. A point defining module in the SRM controlsystem defines a pinned point on a phase current waveform during aninitial current rise phase of the current waveform. The defined pinnedpoint is static with respect to an underlying inductance value of theSRM.

Preferably, there are two options to determine a new pinned point inorder to handle a change in operating conditions and load torqueprofile. The first option depends on the knowledge of inductance valuefor this current for the new operating condition or can be calculated.And the second option is that, if simplifications in the controlmethodology allow, only the slope of the current profile (desiredcurrent rise) over a fixed time period based on this inductance isneeded. The slope of the current (rise) is measured as the waveformreaches the pinned current level.

A slope determining module in the SRM control system determines theslope of the current rise as the current waveform reaches the pinnedpoint. A commutation module in the system is designed to receive theslope of the current rise from the slope determining module and afrequency input signal. The SRM control system further includes an errorcalculating module to calculate an error signal. The SRM control systemis designed to utilize the underlying inductance or the measured currentrise to calculate the error signal. In one configuration, the slope ofthe current rise is utilized to calculate the underlying inductance thatis used to calculate the error signal from the desired inductance. Inanother configuration, the SRM control system is designed to utilize themeasured current rise over a fixed time period to calculate the errorsignal from a desired current rise. The error signal from the calculatedinductance or current slope is used as an input to a control loop in theSRM control system. A time determining module determines an optimum timeto fire a next pulse.

The preferred method describes an overall control architecture of theSRM control system. According to this control architecture, a referencespeed or torque is provided as an input to the system. The slope of thecurrent rise is calculated as the current waveform reaches the pinnedpoint and fed to the commutation module. The underlying inductance valueis calculated utilizing the slope of the current rise. Frequency inputsignal is the other input provided to the commutation module that givesa digital estimate for shaft speed. A current speed is calculatedutilizing the slope of the current rise and frequency input signal. Theerror generator between the reference speed and the current speed isprocessed through a regulator unit which generates a commanded current.The regulator unit may be a proportional-integral (PI) regulator. Thecommanded current is compared with a measured current by an innercurrent loop in the SRM control system. Thereafter, pulse widthmodulation (PWM) signals are generated to create a plurality ofcommutation angles for turning a plurality of switches of the SRM to onand off states utilizing the time signals T_(on), T_(off).

The preferred embodiment includes a method for controlling the SRMutilizing the SRM control system. The method commences by providing theSRM control system. Next, the initial rotor position is provided to theSRM utilizing the initialization mechanism. Then, the pinned point onthe phase current waveform is defined during the initial current risephase of the current waveform. Thereafter, the slope of the current riseis determined as the current waveform reaches the pinned point. Theslope is then fed to the commutation module. Thereafter, the errorsignal from the calculated inductance or current slope is used as aninput to a control loop in the SRM control system. Finally, the timedetermining module determines an optimum time signal to fire the nextpulse. The optimum time signal is fed to SRM for turning the pluralityof SRM switches to on and off states.

Optimum efficiency and greatest load capacity of the SRM is obtainedwhen the pinned point of the current waveform is near the top of theinitial rise of the current waveform and the point on the inductionprofile it is pinned to is near the start of the induction rise for thatphase of the machine.

It is a first objective of the present invention to provide an SRMcontrol system that enables accurate pulse positioning in a sensor-lessenvironment.

A second objective of the present invention is to provide an SRM controlsystem for controlling an SRM that reduces manufacturing imperfectionsand aging effects in the machine.

A third objective of the present invention is to provide an SRM controlsystem adaptable to adjust control parameters for each individualmachine instead of the entire batch of manufactured machines.

A further objective of the present invention is to provide an SRMcontrol system that utilizes simple algorithms for calculating thecurrent rise time.

A still further objective of the present invention is to provide an SRMcontrol system that does not inject additional voltage pulses forcontrolling the switched reluctance electric machine.

These and other advantages and features of the present invention aredescribed with specificity so as to make the present inventionunderstandable to one of ordinary skill in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to enhance their clarity and improve the understanding of thevarious elements and embodiments of the invention, elements in thefigures have not necessarily been drawn. Furthermore, elements that areknown to be common and well understood to those in the industry are notdepicted in order to provide a clear view of the various embodiments ofthe invention. Thus, the drawings are generalized in form in theinterest of clarity and conciseness.

FIG. 1 illustrates a block diagram of a switched reluctance machine(SRM) control system in accordance with the preferred embodiment of thepresent invention;

FIG. 2 illustrates a graphical representation showing change in theinductance profile with respect to change in the electric angle inaccordance with the preferred embodiment of the present invention;

FIG. 3 illustrates an overall control architecture of the SRM controlsystem with speed and current loops in accordance with the preferredembodiment of the present invention;

FIG. 4 illustrates a flowchart of a method for controlling the SRMutilizing the SRM control system in accordance with the preferredembodiment of the present invention;

FIG. 5 illustrates an asymmetric bridge configuration for controllingphase current of the SRM in accordance with the preferred embodiment ofthe present invention; and

FIG. 6 illustrates a pinned point at a current waveform of a three-phaseSRM in accordance with the preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following discussion that addresses a number of embodiments andapplications of the present invention, reference is made to theaccompanying drawings that form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. It is to be understood that other embodiments may beutilized, and changes may be made without departing from the scope ofthe present invention.

Various inventive features are described below that can each be usedindependently of one another or in combination with other features.However, any single inventive feature may not address any of theproblems discussed above or only address one of the problems discussedabove. Further, one or more of the problems discussed above may not befully addressed by any of the features described below.

As used herein, the singular forms “a”, “an” and “the” include pluralreferents unless the context clearly dictates otherwise. “And” as usedherein is interchangeably used with “or” unless expressly statedotherwise. As used herein, the term ‘about” means+/−5% of the recitedparameter. All embodiments of any aspect of the invention can be used incombination, unless the context clearly dictates otherwise.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words ‘comprise’, ‘comprising’, and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to”. Words using the singular or pluralnumber also include the plural and singular number, respectively.Additionally, the words “herein,” “wherein”, “whereas”, “above,” and“below” and words of similar import, when used in this application,shall refer to this application as a whole and not to any particularportions of the application.

The description of embodiments of the disclosure is not intended to beexhaustive or to limit the disclosure to the precise form disclosed.While the specific embodiments of, and examples for, the disclosure aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the disclosure, as thoseskilled in the relevant art will recognize.

FIG. 1 illustrates a block diagram of a switched reluctance machine(SRM) control system 10 for controlling an SRM 26. The SRM controlsystem 10 enables adaptive pulse positioning over a wide range of speedsand loads. The SRM control system 10 includes an initialization module12 to provide an initial rotor position for the SRM 26 utilizing aninitialization mechanism. In the preferred embodiment, theinitialization mechanism is adaptable to implement several approacheslike hard alignment or any other mathematical approach. A point definingmodule 14 in the SRM control system 10 defines a pinned point on a phasecurrent waveform during an initial current rise phase of the currentwaveform. The defined pinned point is static with respect to anunderlying inductance value of the SRM 26 is a function of desiredoperating point. The PI controller for the speed loop controls theamount of time from x (pinned point) to the turn on of the next phase.If demand for speed changes then the demand for current also changes.This means that the slopes are different and requires change in thepinned point. The pinned point is defined at a specifically chosenmagnitude of current between 50% and 100% of the steady state current onthe initial current rise, particularly when there is a sudden change inthe operating condition (load torque). This can also be useful inimproving accuracy as we enter single pulse mode and the currentwaveform begins to plateau as the waveform gets closer to the alignedposition. The goal is to have the pinned point low enough or far enoughfrom the curved profile of current.

Preferably, there are two options to determine a new pinned point inorder to handle a change in operating conditions and load torqueprofile. The first option depends on the knowledge of inductance valuefor this current for the new operating condition or can be calculated.And the second option is that, if simplifications in the controlmethodology allow, only the slope of the current profile (desiredcurrent rise) over a fixed time period based on this inductance isneeded.

A slope determining module 16 determines a slope 42 (see FIG. 3) of thecurrent rise as the current waveform reaches the pinned point. The slopeof the current (rise) is measured as the waveform reaches the pinnedcurrent level. As shown in the graphical representation illustrated inFIG. 2, if we change the current value, the inductance profile alsochanges with it. This means the angle corresponding to the pinnedposition must be changed until the same slope is arrived as was arrivedat the previous case.

A commutation module 18 is designed to receive the slope 42 of thecurrent rise from the slope determining module 16. The SRM controlsystem 10 further comprises an error calculating module 20 to calculatean error signal. The SRM control system 10 is designed to utilize theunderlying inductance or the measured current rise to calculate theerror signal. In one configuration, the slope 42 of the current rise isutilized to calculate the underlying inductance which is used tocalculate the error signal from the desired inductance. In anotherconfiguration, the SRM control system 10 is designed to utilize themeasured current rise over a fixed time period to calculate the errorsignal from a desired current rise. The error signal from the calculatedinductance or current slope is used as an input to a control loop 22 inthe SRM control system 10. Finally, a time determining module 24determines an optimum time T_(on), T_(off) 40 (see FIG. 3) to fire anext pulse. The optimum time T_(on), T_(off) 40 turns a plurality ofswitches of the SRM 26 to on and off states. In one configuration of thepreferred embodiment, position is determined to fire a next pulse.

FIG. 3 shows an overall control architecture of the SRM control system10 with speed and current loops. Here, a reference speed (Ref Speed) 32or torque is provided as an input to the system 10. Preferably, theproposed method for controlling SRM 26 utilizes a current feedback. Theslope 42 of the current rise is calculated as the current waveformreaches the pinned point and is fed to the commutation module 18. Theunderlying inductance value is calculated utilizing the slope 42 of thecurrent rise. Frequency input signal T_(p) 44 is the other inputprovided to the commutation module 18 that gives a digital estimate forshaft speed. A current speed 36 is calculated utilizing the slope 42 ofthe current rise and frequency input signal T_(p) 44. The errorgenerator between the reference speed 32 and the current speed 36 isprocessed through a regulator unit 30 that generates a commanded current(I_(ced)) 34. The regulator unit 30 may be a proportional-integral (PI)regulator. The commanded current 34 is compared with a measured current(I_(phase)) 38 by an inner current loop in the SRM control system 10 togenerate pulse width modulation (PWM) signals. The PWM signals create aplurality of commutation angles for turning the plurality of switches ofthe SRM 26 to on and off states utilizing the time signals T_(on),T_(off) 40.

FIG. 4 shows a flowchart of a method for controlling the SRM 26utilizing the SRM control system 10. As shown in block 50, the SRMcontrol system having the commutation module is provided. Next, theinitial rotor position is provided to the SRM utilizing theinitialization mechanism as shown in block 52. Then, the pinned point onthe phase current waveform is defined during the initial current risephase of the current waveform as indicated at block 54. Thereafter, theslope of the current rise is determined as the current waveform reachesthe pinned point as shown in block 56. The slope is then fed to thecommutation module. Thereafter, the error signal from the calculatedinductance or current slope is used as an input to a control loop in theSRM control system as shown in block 58. Finally, the time determiningmodule determines an optimum time signal to fire the next pulse asindicated at block 60. The optimum time signal is fed to SRM for turningthe plurality of SRM switches to on and off states.

FIG. 5 shows an asymmetric bridge configuration typically used forcontrolling the SRM 26. This configuration has each phase connectedbetween two switches T1, T2 which allows independent control and ensuresthat the inverter does not have a shoot-through failure. The turn-on andturn-off signals 40 in FIG. 3 are used to control switches T1 and T2.

FIG. 6 shows three current waveforms for a three-phase machine. In thisexample, “x” is the pinned point of the current waveform in phase A ofthe machine. Here, the pinned point is roughly at 80% of steady statecurrent for the operating condition. Optimum efficiency and greatestload capacity of the SRM is obtained when the pinned point of thecurrent waveform is near the top of the initial rise of the currentwaveform and the point on the induction profile it is pinned to is nearthe start of the induction rise for that phase of the machine.

In the current embodiment, the feedback from one commutation pulse isused for the positioning of the next pulse. Instead, the feedback fromthis pulse could be used to adjust the position of the next pulse in thesame phase, or the next time that specific stator rotor pole combinationis reached, or anything in between.

Using each pulse to modify only pulses of the same phase has the benefitof allowing phases to be adjusted independently due to non-uniforminductance on each phase; however, the position feedback is slower by amultiple of the number of phases in the machine. This could be overcomeby using the error of the current pulse to input to two control loops.Among the two control loops, one adjusts the current phase and the otheradjusts all of the phases allowing for both minor adjustments betweenphases while still achieving rapid feedback to the main controlmethodology.

Using each pulse to modify only the same stator rotor poll combinationhas the benefit that it allows adjustments to non-uniform polepositions, air-gap and inductance; however, this position feedback isslower by a multiple of the number of phases times the number of rotorpoles. A similar methodology to the previous could be used to introducethe extra degree of freedom while still maintaining rapid feedback.

In the current embodiment, an event base control loop was utilized. Anyform of control loop operating from the error between the desiredinductance (or desired current rise) and the measured inductance (ormeasured current rise) meets the intent of the preferred embodiment.

In the current embodiment, the current was pinned on the initial risingedge of the pulse; however, any point along an arbitrary waveform can beused as the pinned point.

In the current embodiment, the current rise was used at the specifiedpoint on the current rise; however, at the desired waveform position,the phase could be switched off or freewheeled and the currentdrop/decay at that point could be used in the same manner to controlposition.

In the current embodiment, the output of the control loop is the desiredtime between pulses and when the time from the last pulse is reached,the next pulse is fired. The output of the control loop could also betuned such that it is the desired position on a software encoder whichis being updated continuously based on the speed estimations. Thismethodology induces further error because the software encoder is proneto drift due to error in the speed measurements but would achieve thesame effect. Similarly, a hardware encoder could be used and thismethodology could position the pulses relative to the hardware encoder.

This methodology could be extended further to allow for adjustments inthe desired inductance (or desired current rise) based on speed, load,or desired optimization. These adjustments could be applied from alookup table based on current operating point or calculated real timebased on an adjustment formula.

The foregoing description of the preferred embodiment of the presentinvention has been presented for the purpose of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teachings. It is intendedthat the scope of the present invention not be limited by this detaileddescription, but by the claims and the equivalents to the claimsappended hereto.

What is claimed is:
 1. A method for controlling a switched reluctancemachine (SRM) comprising the steps of: a) providing an SRM controlsystem having a commutation module; b) providing an initial rotorposition for the SRM utilizing an initialization mechanism; c) defininga pinned point on a phase current waveform during an initial currentrise phase of the current waveform; d) determining a slope of thecurrent rise as the current waveform reaches the pinned point, the slopebeing fed to the commutation module; e) calculating an error signal andproviding the error signal as an input to a control loop in the SRMcontrol system; and f) determining an optimum time to fire a next pulse;whereby the SRM control system enables adaptive pulse positioning over awide range of speeds and loads.
 2. The method of claim 1 wherein thedefined pinned point is static with respect to an underlying inductancevalue of the SRM.
 3. The method of claim 1 wherein the pinned positionis defined at a point corresponding to a magnitude between 50% and 100%of the steady state current on the initial current rise.
 4. The methodof claim 1 wherein the SRM control system is designed to utilize theunderlying inductance to calculate the error signal from a desiredinductance value.
 5. The method of claim 1 wherein the SRM controlsystem is designed to utilize the measured current rise over a fixedtime period to calculate the error signal from a desired current rise.6. A method for controlling a switched reluctance machine (SRM)comprising the steps of: a) providing an SRM control system having acommutation module; b) providing an initial rotor position for the SRMutilizing an initialization mechanism; c) defining a pinned point on aphase current waveform during an initial current rise phase of thecurrent waveform, the pinned point being static with respect to anunderlying inductance value of the SRM; d) determining a slope of thecurrent rise as the current waveform reaches the pinned point, the slopebeing fed to the commutation module; e) providing a frequency inputsignal to the commutation module to obtain a digital estimate for shaftspeed; f) calculating a current speed utilizing the slope of the currentrise and frequency input signal; g) generating a commanded current by aregulator unit utilizing a reference speed and the current speed; h)comparing the commanded current with a measured current by an innercurrent loop in the SRM control system; and i) generating pulse widthmodulation (PWM) signals to create a plurality of commutation angles forturning a plurality of switches of the SRM to on and off states; wherebythe SRM control system enables adaptive pulse positioning over a widerange of speeds and loads.
 7. The method of claim 6 wherein the pinnedposition is defined at a point corresponding to a magnitude between 50%and 100% of the steady state current on the initial current rise.
 8. Themethod of claim 6 herein the regulator unit is a proportional-integralunit.
 9. A switched reluctance machine (SRM) control system forcontrolling an SRM comprises: an initialization module to provide aninitial rotor position for the SRM utilizing an initializationmechanism; a point defining module to define a pinned point on a phasecurrent waveform during an initial current rise phase of the currentwaveform; a slope determining module to determine a slope of the currentrise as the current waveform reaches the pinned point; a commutationmodule to receive the slope of the current rise from the slopedetermining module and a frequency input signal; an error calculatingmodule to calculate an error signal that being fed to a control loop inthe SRM control system; and a time determining module to determine anoptimum time to fire a next pulse; whereby the SRM control systemenables adaptive pulse positioning over a wide range of speeds andloads.
 10. The SRM control system of claim 9 wherein the defined pinnedpoint is static with respect to an underlying inductance value of theSRM.
 11. The SRM control system of claim 9 wherein the pinned positionis defined at a point corresponding to a magnitude between 50% and 100%of the steady state current on the initial current rise.
 12. The SRMcontrol system of claim 9 is designed to utilize the underlyinginductance to calculate the error signal from a desired inductancevalue.
 13. The SRM control system of claim 9 is designed to utilize themeasured current rise over a fixed time period to calculate the errorsignal from a desired current rise.
 14. The SRM control system of claim9 wherein the optimum time determined from the time determining moduleis utilized to turn the plurality of switches of the SRM to on and offstates.